The present invention relates to bags for packaging meat and, in particular, to heat-shrinkable bags having a protective patch for packaging bone-in primal cuts of meat.
The use of bags formed of heat-shrinkable thermoplastic film for packaging primal cuts of meat is well known in the art. In use, the primal cut is loaded into the bag. The bag is evacuated and heat sealed to form a closed package. Next the package is exposed to warm water or other heating means to cause the bag to shrink and form fit the primal cut.
The bag film is relatively thin and usually not more than about 4 mils (0.10 mm) and generally less than 3 mils (0.076 mm) thick. This thin a bag generally is not satisfactory for packaging cuts of meat which contain bone. For example, the ribs or other sharp bone protrusions as contained by primal and subprimal rib beef cuts or pork ribs, may puncture the bag during the evacuation of air from the bag or during heat shrinking as the bag draws tightly about the bone-in meat cut. A puncture in the bag is undesirable as it allows the meat in the area of the puncture to be exposed to oxygen which shortens shelf life of the packaged product. The problem of bone punctures further is compounded by abrasion between adjacent packages caused by vibration and movement of the meat packages one against another during transport and handling.
A common solution to the problem is to improve the puncture and abrasion resistance of the bag film by adhering a patch to the outer surface of the heat-shrinkable bag. U.S. Pat. No. 4,755,403 discloses use of an oriented, heat-shrinkable multilayer patch which is affixed by adhesive to one surface of an oriented multilayer heat-shrinkable bag. U.S. Pat. No. 5,545,419 discloses adhering two patches to the bag, one to each outer surface of the flattened bag. The shrink properties of the patch in each case are matched to the shrink properties of the bag to reduce the likelihood of delamination of the patch from the bag during heat shrinking.
The shrink properties of the film from which the patch and bag are made derive from the process of orienting the patch (or bag) film. Orientation to provide heat-shrinkability at temperatures below 100xc2x0 C. and preferably about 90xc2x0 C. or less is well known in the art. Orientation also is known to improve the puncture resistance of the film. Briefly, orientation is provided by extruding a relatively thick walled tube. The tube is first cooled then reheated to soften it so that it can be expanded by inflation. Inflation provides transverse orientation. Orientation in the machine direction is provided by drawing the tube forward as it is expanded. The tube is formed into bags by heat sealing across the tube or the tube is slit lengthwise and cut to form individual biaxially oriented patches.
While orientation contributes to the puncture resistance of the film, abrasion resistance is a function of film thickness and patches on the order of 4 to 8 mils (0.10 to 0.20 mm) are commonly found on bags which have a wall thickness of 3 mils (0.076 mils) or less.
While a relatively thick patch film is desirable for abrasion resistance, it is known that thick films are more difficult to orient than thin films. In general, as film thickness increases above about 3 mils (0.076 mm) it becomes more difficult to orient so those properties, such as improved puncture resistance which benefit from orientation, begin to decrease. Thus, the desirable properties for a patch film of abrasion and puncture resistance represent a compromise. The compromise is to provide a film thin enough to benefit from the orientation process by having enhanced puncture resistance, yet thick enough to provide enhanced abrasion resistance.
In order to optimize the benefits of puncture resistance through orientation and abrasion resistance by increasing film thickness, multilayer patch materials are used in the prior art. U.S. Pat. Nos. 4,755,403 and 5,545,419 disclose a heat-shrinkable irradiated patch material formed by collapsing an oriented multilayer tube. The tube wall has an inner surface formed of a self-adhering layer of ethylene vinyl acetate (EVA) and an outer surface formed of a polyethylene-EVA blend, the polyethylene being linear low density polyethylene (LLDPE) such as DOWLEX 2045 from the Dow Chemical Company. When the oriented tube is collapsed, the inner surfaces of the tube adhere together and in effect form a three layer patch having the general structure EVA+LLDPE/EVA/EVA+LLDPE. The irradiation dosage is about 7 MR. In a commercial version the irradiated patch is about 5 mils (0.127 mm) thick and is adhered to a multilayer bag having a wall thickness of about 2.3 mils (0.058 mm). Accordingly, in this commercial version the multilayer patch is over twice the thickness of the bag film.
U.S. Pat. No. 5,302,402 discloses a monolayer patch formed of a blown film. The film is not oriented so it is not heat shrinkable. The patch film is composed of a blend of EVA and very low density polyethylene (VLDPE) such as a Dow type XU 61520.01 which has a density of about 0.912 g/cc. As disclosed in this patent, the nonoriented patch of about 5 mils (0.127 mm) to 7 mils (0.178 mm) is applied to the surface of a three layered bag having a wall thickness as thin as 2.25 mils (0.057 mm). The outer surface of the bag is composed of a polymer blend including EVA and the sole bonding means between the nonshrinkable patch and the shrinkable bag is a high surface energy of at least 38 dynes/cm wetting tension. The high surface energy is provided by corona treatment. Prior to heat shrinking, the high surface energy allows the patch to cling to the bag in that it is still possible to peel the patch from the bag. After heat shrinking, however, it was found that the bond between the nonshrunk patch and the shrunk bag increased such that a bond was created sufficient to prevent delamination of the patch from the bag. The abrasion resistance of the bag as measured by simulated and actual shipping tests was comparable to commercial bags having heat-shrinkable patches. Thus, the bag as described in the ""402 Patent provided abrasion resistance using a relatively thick 5 to 7 mil (0.127 to 0.178 mm) nonoriented monolayer patch and was said to eliminate the need to biaxially orient and irradiate the patch film.
Since the patch was not oriented, it could be made thicker to improve abrasion resistance. Thus, while the bag as disclosed in the ""402 Patent exhibited certain advantages, the lack of orientation of the patch film compromises puncture resistance and the patch generally was thicker than an oriented patch.
Accordingly, an object of the present invention is to provide an improved patch bag for packaging bone-in primal and subprimal cuts of meat.
Another object is an improved patch bag having abrasion resistance and enhanced puncture resistance.
A further object is an improved patch bag which provides abrasion resistance and has enhanced puncture resistance while utilizing a relatively thin patch.
Yet another object is an improved patch bag wherein abrasion resistance and enhanced puncture resistance are achieved using a relatively thin monolayer patch.
In one aspect, the present invention relates to a patch bag for packaging bone-in meat cuts and especially bone-in primal and subprimal meat cuts such as beef ribs or pork loins and the like. The patch bag comprises a relatively thin walled bag formed of a biaxially oriented multilayered thermoplastic film and a relatively thin patch adhered to an outer surface of the bag, the patch being formed of a biaxially oriented monolayer film.
The bag preferably is formed of a multilayer barrier film wherein an oxygen barrier layer is disposed between a heat-sealable layer which defines the inner surface of the bag, and an abuse layer which defines the outer surface of the bag.
The film comprising the bag structure is biaxially oriented and has a total thickness of between 2.25 mils (0.057 mm) and 3.5 mils (0.089 mm). The patch adhered to the outer surface of the bag is a monolayer biaxially oriented film substantially less than 5 mils (0.127 mm) thick and preferably no more than about 3.5 mils (0.089 mm). Accordingly, the bag film and patch film are about equal in thickness so a feature of the present invention is that the patch is relatively thin and not substantially thicker than the thickness of the bag. Further, the patch according to the present invention provides abrasion resistance and enhanced puncture resistance together with the economy of reducing the patch thickness by 40% or more of the thickness of conventional patches.
The patch bag of the present invention may be formed by laminating the patch to the bag film with a suitable adhesive. However, in a preferred aspect, the invention is characterized by the oriented patch being adhered to the outer surface of the oriented bag film without adhesive. In this respect, one surface of the patch and an outer bag surface are both corona treated to raise the surface energy such that the patch will cling to the bag. This cling is sufficient to maintain the patch in position through the packaging process. Thereafter, upon heat shrinking a still greater adhesion occurs between the oriented patch and bag to the extent that delamination of the patch from the bag during handling and transport of the packaged meat does not occur. The adhesion of an unoriented patch to the bag using corona treatment to cause the adhesion is known from U.S. Pat. No. 5,302,402. However, accomplishing the adhesion using an oriented patch was unexpected in that it was not known whether corona treatment of an oriented film surface would have the desired effect. Not only was the adhesion unexpected, the adherence of the oriented patch according to the present invention proved to be even greater than that of the prior art unoriented patch. Further, adhesion was accomplished even without the surface roughness which was found to be essential for adhering an unoriented patch to an oriented film. Surface roughness can result from melt fracture during extrusion of the film. In the case of an unoriented blown film patch, it was found that films which exhibited little or no melt fracture exhibited poor adherence after corona discharge. In the case of the oriented patch according to the present invention, the absence of melt fracture is no deterrent to adhesion. This means a clear patch film can be made since melt fracture detracts from film clarity.
In one embodiment, the present invention is characterized by the formulation of the monolayer patch being a blend of primarily four components including an EVA and three different ethylene a-olefin copolymers. A first of the ethylene xcex1-olefin copolymers preferably has a melting point of between 55 to 90xc2x0 C., the second has a melting point of between 90 to 110xc2x0 C. and the third has a melting point of between 115 to 130xc2x0 C. These same four components make up the heat seal layer of the bag film. Accordingly, in one embodiment of the patch bag of the present invention, the heat seal layer of the bag film and the patch film are substantially the same formulation.
In an alternative and preferred embodiment, the bag film is the same but the EVA is omitted from the patch film.